Simply changing the amount entered in box 2a without submitting a substitute Form 1099-R runs the risk of the IRS detecting the difference between what the IRS calculates from the Form 1099-R provided to them by the plan and what was reported on the tax return, causing the IRS to "correct" the tax return and issue a refund. A substitute Form 1099-R (Form 4852) is the defined method by which one disagrees with what the plan reported and is where the explanation belongs.
